Database - Index

https://stackoverflow.com/questions/24943436/how-to-create-an-index-with-jpa-hibernate-and-use-fields-from-mappedsupperclass

https://blog.csdn.net/m0\_37204491/article/details/72852996

index 就是建立一個 B+ 樹，讓查詢更快速，此樹會存在硬碟裡

NoSQL 一般不支持 multi-index